1. Field of the Invention
The present invention generally relates to a balanced-to-unbalanced transformer embedded with a filter and, more particularly, to a balanced-to-unbalanced transformer embedded with a filter using a multi-layered substrate. In the balanced-to-unbalanced transformer, there are vertically coupled transmission lines designed in different layers in the multi-layer substrate to increase transmission performances. A capacitor and a transmission line are connected to a single-ended I/O port of the balanced-to-unbalanced transformer such that impedance matching is achieved and a band-pass filter is embedded in the balanced-to-unbalanced transformer.
2. Description of the Prior Art
In recent years, chip circuits with a balanced output have attracted tremendous attention in wireless communication applications such as power amplifiers, radio frequency (RF) transceivers because the differential output circuits help to resist the high-frequency noise. The purpose of a balanced-to-unbalanced transformer (balun) is to transform an unbalanced signal in the wireless transceivers to a pair of balanced differential signals that have the same amplitude and are 180 degrees out-phased. Therefore, the common-mode noise is prevented. It can also be realized to transform a pair of balanced differential signals to an unbalanced signal. In addition to the afore-mentioned balanced-to-unbalanced transformer, the filter is another key element to filter out the undesired noise signal so as to improve transmission quality.
The balanced-to-unbalanced transformer and the filter can be implemented using discrete surface mounted device (SMD) with capacitors and inductors. However, in the circuit using SMD elements, the characteristics of the real elements may be different from that of the designed elements. On the other hand, open circuits may occur due to improper soldering for the discrete passive elements. Accordingly, the state-of-the-art balanced-to-unbalanced transformer and filter are no longer designed using discrete passive elements.
In the U.S. Pat. No. 6,803,835 entitled “Integrated Filter Balun”, a conventional balanced-to-unbalanced transformer integrated with a filter is disclosed using ladder-type and lattice-type filters with distributed ¼ wavelength and ¾ wavelength transmission lines. Even though U.S. Pat. No. 6,803,835 provides a balanced-to-unbalanced transformer integrated with a filter with a small area equal to that of a conventional balanced-to-unbalanced transformer, it requires more elements to implement a filter.
In addition, in the U.S. Pat. No. 6,791,431 entitled “Compact Balun with Rejection Filter for 802.11A and 802.11B Simultaneous Operation”, a 180 degree hybrid circuit with a band reject filter is provides to achieve a balanced-to-unbalanced transformer. However, the balanced-to-unbalanced transformer using the hybrid circuit requires a 50-ohm resistor and ⅔ wavelength transmission lines.
In “LTCC-MLC Chip-type Balun Realised by LC Resonance Method” published in Electronics Letters, May 23, 2002, vertically coupled transmission lines are utilized in the balanced-to-unbalanced transformer. However, the circuit structure does not include the balanced-to-unbalanced transformer with a filter.
Therefore, vertically coupled transmission lines and a multi-layered substrate are never applied in a balanced-to-unbalanced transformer embedded with a filter. Therefore, the conventional balanced-to-unbalanced transformer embedded with a filter suffers from a large size.
Therefore, there exists a need in providing a balanced-to-unbalanced transformer embedded with a filter using a multi-layered substrate. In the balanced-to-unbalanced transformer, there are vertically coupled transmission lines designed in different layers in the multi-layer substrate to increase transmission performances. A capacitor and a transmission line are connected to a single-ended I/O port of the balanced-to-unbalanced transformer such that impedance matching is achieved and a band-pass filter is embedded in the balanced-to-unbalanced transformer.